Redis主从复制

Redis Sentinel实现原理分析这篇文。Sentinel是为主从复制服务的,所以在这篇文章里面,我们反过来讲一下主从复制的实现。

主从复制涉及到RDB等机制,其中持久化部分在Redis持久化机制实现中介绍。

Redis主从复制流程简介

Redis Sentinel是对主从复制流程而言的,所以我们先要理解主从复制的大概流程。这里需要注意,主从复制并不是Redis Cluster。

  1. Slave接受到SLAVEOF命令
  2. Slave连接Master
  3. Slave PING Master
  4. 鉴权
  5. Slave发送SYNC/PSYNC命令

PSYNC命令用法

PSYNC命令如下所示,其中:

  1. replicationid表示我们断线重连前Master服务器的id
  2. offset表示Slave接受到最后命令的偏移量,以字节计算
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// 旧
PSYNC runid offset
// 新
PSYNC replicationid offset

这里还有个特殊用法,表示我们要触发一次全量复制。

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PSYNC ? -1

在Redis 2.8后,提供了PSYNC,这个命令能够支持全量复制和部分复制。这样在Slave断线重连之后,就可以部分复制,从而节省Master的计算资源和带宽。
在Redis 4.0版本后,优化了增量复制,主要包括:

  1. 重启后,也可以进行部分复制
    之前这种情况,重启后会丢失runid,从而触发PSYNC ? -1
  2. 当Slave被promote称为Master后,其他Slave可以从新Master处复制

主要流程

主要类以及常数

在服务器类中定义了masterhost,表示Master节点的地址。如果是NULL,表示自己就是Master。所以这个字段会被用来判断是MAster还是Slave

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// server.h
struct redisServer {
char *masteruser; /* AUTH with this user and masterauth with master */
char *masterauth; /* AUTH with this password with master */
char *masterhost; /* Hostname of master */
int masterport; /* Port of master */
}

介绍下repl_state的状态:

  1. REPL_STATE_NONE 0
    表示现在是SLAVEOF NO ONE的
  2. REPL_STATE_CONNECT 1
    replicationCron判断,如果处于这个状态,表示现在要去尝试连接Master了。
  3. REPL_STATE_CONNECTING 2

下面的状态是握手过程中的状态:

  1. REPL_STATE_RECEIVE_PONG 3
  2. REPL_STATE_SEND_AUTH 4
  3. REPL_STATE_RECEIVE_AUTH 5
  4. REPL_STATE_SEND_PORT 6
  5. REPL_STATE_RECEIVE_PORT 7
  6. REPL_STATE_SEND_IP 8
  7. REPL_STATE_RECEIVE_IP 9
  8. REPL_STATE_SEND_CAPA 10
  9. REPL_STATE_RECEIVE_CAPA 11
  10. REPL_STATE_SEND_PSYNC 12
  11. REPL_STATE_RECEIVE_PSYNC 13

下面状态是握手完毕的状态:

  1. REPL_STATE_TRANSFER 14
  2. REPL_STATE_CONNECTED 15

连接建立流程

  1. connectWithMaster

Full Sync流程

Partial Sync流程

代码解释 Slave部分

connectWithMaster: 建立套接口连接

connConnect系列函数,以及connection类封装了有关网络连接的库。
实际上connConnect通过anetTcpNonBlockBestEffortBindConnect尝试建立一个非阻塞的套接字,此时connect函数可能返回EINPROGRESS表示连接还在建立过程中,但我们其实可以不用等。通过aeCreateFileEvent将这个Socket描述符加入到事件循环里面,等到这个套接字可以写之后,会触发对应的回调。
等到连接建立后,回调会通过connSocketEventHandler被唤起。

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int connectWithMaster(void) {
server.repl_transfer_s = server.tls_replication ? connCreateTLS() : connCreateSocket();
if (connConnect(server.repl_transfer_s, server.masterhost, server.masterport,
NET_FIRST_BIND_ADDR, syncWithMaster) == C_ERR) {
serverLog(LL_WARNING,"Unable to connect to MASTER: %s",
connGetLastError(server.repl_transfer_s));
connClose(server.repl_transfer_s);
server.repl_transfer_s = NULL;
return C_ERR;
}


server.repl_transfer_lastio = server.unixtime;
server.repl_state = REPL_STATE_CONNECTING;
serverLog(LL_NOTICE,"MASTER <-> REPLICA sync started");
return C_OK;
}

syncWithMaster: 握手以及准备传输RDB

在连接完毕后,connectWithMaster会回调syncWithMaster,此时状态是REPL_STATE_CONNECTING

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void syncWithMaster(connection *conn) {
char tmpfile[256], *err = NULL;
int dfd = -1, maxtries = 5;
int psync_result;

检查一下,如果现在又是SLAVEOF NO ONE了,就把这个连接关掉。

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/* If this event fired after the user turned the instance into a master
* with SLAVEOF NO ONE we must just return ASAP. */
if (server.repl_state == REPL_STATE_NONE) {
connClose(conn);
return;
}

因为是非阻塞的连接,所以我们要检查一下现在连接的状态。如果失败,就goto error,里面内容是重置状态,例如,server.repl_state会被重置为REPL_STATE_CONNECT

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/* Check for errors in the socket: after a non blocking connect() we
* may find that the socket is in error state. */
if (connGetState(conn) != CONN_STATE_CONNECTED) {
serverLog(LL_WARNING,"Error condition on socket for SYNC: %s",
connGetLastError(conn));
goto error;
}

下面是一个状态机的实现。我们在Sentinel中已经见过类似的了,Redis中状态机的实现就是,对于状态X,表示状态X前一个状态已经处理完了,目前正在处理状态X的工作。当状态机处理完一个状态后,在最后将状态设置为下一个要做的事情。也就是我们不用类似X_FINISHED这样的状态,因为X_FINISHED根据完成的情形不同,可能有多种状态转移
【REPL_STATE_CONNECTING】这个状态下,我们尝试发送一个同步命令PING,然后直接修改状态到REPL_STATE_RECEIVE_PONG。如果这个同步命令发送有问题,就直接goto error了,不会走到下面流程。

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/* Send a PING to check the master is able to reply without errors. */
if (server.repl_state == REPL_STATE_CONNECTING) {
serverLog(LL_NOTICE,"Non blocking connect for SYNC fired the event.");
/* Delete the writable event so that the readable event remains
* registered and we can wait for the PONG reply. */
connSetReadHandler(conn, syncWithMaster);
connSetWriteHandler(conn, NULL);
server.repl_state = REPL_STATE_RECEIVE_PONG;
/* Send the PING, don't check for errors at all, we have the timeout
* that will take care about this. */
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"PING",NULL);
if (err) goto write_error;
return;
}

【REPL_STATE_RECEIVE_PONG】我们只要收到对PING的回复,就进入了REPL_STATE_RECEIVE_PONG状态,但这个回复未必是PONG,也可能是一个AUTH错误。

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/* Receive the PONG command. */
if (server.repl_state == REPL_STATE_RECEIVE_PONG) {
err = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);

/* We accept only two replies as valid, a positive +PONG reply
* (we just check for "+") or an authentication error.
* Note that older versions of Redis replied with "operation not
* permitted" instead of using a proper error code, so we test
* both. */
if (err[0] != '+' &&
strncmp(err,"-NOAUTH",7) != 0 &&
strncmp(err,"-NOPERM",7) != 0 &&
strncmp(err,"-ERR operation not permitted",28) != 0)
{
serverLog(LL_WARNING,"Error reply to PING from master: '%s'",err);
sdsfree(err);
goto error;
} else {
serverLog(LL_NOTICE,
"Master replied to PING, replication can continue...");
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_AUTH;
}

【REPL_STATE_SEND_AUTH】如果需要AUTH认证,我们就发送AUTH,进入REPL_STATE_RECEIVE_AUTH。否则直接进入REPL_STATE_SEND_PORT

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/* AUTH with the master if required. */
if (server.repl_state == REPL_STATE_SEND_AUTH) {
if (server.masteruser && server.masterauth) {
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"AUTH",
server.masteruser,server.masterauth,NULL);
if (err) goto write_error;
server.repl_state = REPL_STATE_RECEIVE_AUTH;
return;
} else if (server.masterauth) {
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"AUTH",server.masterauth,NULL);
if (err) goto write_error;
server.repl_state = REPL_STATE_RECEIVE_AUTH;
return;
} else {
server.repl_state = REPL_STATE_SEND_PORT;
}
}

【REPL_STATE_RECEIVE_AUTH】如果验证通过,就进入REPL_STATE_SEND_PORT

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/* Receive AUTH reply. */
if (server.repl_state == REPL_STATE_RECEIVE_AUTH) {
err = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
if (err[0] == '-') {
serverLog(LL_WARNING,"Unable to AUTH to MASTER: %s",err);
sdsfree(err);
goto error;
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_PORT;
}

【REPL_STATE_SEND_PORT】下面一步,我们需要发送我们当前的端口,进入REPL_STATE_RECEIVE_PORT状态。
在发送完之后,我们在主节点执行INFO replication,会在其中显示我们反馈的port。
【Q】slave_announce_portport的区别是什么?

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/* Set the slave port, so that Master's INFO command can list the
* slave listening port correctly. */
if (server.repl_state == REPL_STATE_SEND_PORT) {
int port;
if (server.slave_announce_port) port = server.slave_announce_port;
else if (server.tls_replication && server.tls_port) port = server.tls_port;
else port = server.port;
sds portstr = sdsfromlonglong(port);
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"REPLCONF",
"listening-port",portstr, NULL);
sdsfree(portstr);
if (err) goto write_error;
sdsfree(err);
server.repl_state = REPL_STATE_RECEIVE_PORT;
return;
}

【REPL_STATE_RECEIVE_PORT】接下来,我们用类似的办法发送IP,这里注意,如果没有指定slave_announce_ip就直接跳转到REPL_STATE_SEND_CAPA,否则跳转到REPL_STATE_SEND_IP

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/* Receive REPLCONF listening-port reply. */
if (server.repl_state == REPL_STATE_RECEIVE_PORT) {
err = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF listening-port. */
if (err[0] == '-') {
serverLog(LL_NOTICE,"(Non critical) Master does not understand "
"REPLCONF listening-port: %s", err);
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_IP;
}

/* Skip REPLCONF ip-address if there is no slave-announce-ip option set. */
if (server.repl_state == REPL_STATE_SEND_IP &&
server.slave_announce_ip == NULL)
{
server.repl_state = REPL_STATE_SEND_CAPA;
}

/* Set the slave ip, so that Master's INFO command can list the
* slave IP address port correctly in case of port forwarding or NAT. */
if (server.repl_state == REPL_STATE_SEND_IP) {
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"REPLCONF",
"ip-address",server.slave_announce_ip, NULL);
if (err) goto write_error;
sdsfree(err);
server.repl_state = REPL_STATE_RECEIVE_IP;
return;
}

/* Receive REPLCONF ip-address reply. */
if (server.repl_state == REPL_STATE_RECEIVE_IP) {
err = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF listening-port. */
if (err[0] == '-') {
serverLog(LL_NOTICE,"(Non critical) Master does not understand "
"REPLCONF ip-address: %s", err);
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_CAPA;
}

【REPL_STATE_SEND_CAPA】发送IP的过程很类似,我们就不说了。下面这一对状态是REPL_STATE_SEND_CAPA,用来发送Slave的容量。这一对状态结束之后,进入REPL_STATE_SEND_PSYNC状态。
【Q】这个容量指的是什么?

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/* Inform the master of our (slave) capabilities.
*
* EOF: supports EOF-style RDB transfer for diskless replication.
* PSYNC2: supports PSYNC v2, so understands +CONTINUE <new repl ID>.
*
* The master will ignore capabilities it does not understand. */
if (server.repl_state == REPL_STATE_SEND_CAPA) {
err = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"REPLCONF",
"capa","eof","capa","psync2",NULL);
if (err) goto write_error;
sdsfree(err);
server.repl_state = REPL_STATE_RECEIVE_CAPA;
return;
}

/* Receive CAPA reply. */
if (server.repl_state == REPL_STATE_RECEIVE_CAPA) {
err = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF capa. */
if (err[0] == '-') {
serverLog(LL_NOTICE,"(Non critical) Master does not understand "
"REPLCONF capa: %s", err);
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_PSYNC;
}

【REPL_STATE_SEND_PSYNC】下面,我们尝试PSYNC。主要就是调用若干次slaveTryPartialResynchronization:第一次传0进去,让它发PSYNC指令,并且设置状态为REPL_STATE_RECEIVE_PSYNC;后面就不断地传1进去,查询结果。

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/* Try a partial resynchonization. If we don't have a cached master
* slaveTryPartialResynchronization() will at least try to use PSYNC
* to start a full resynchronization so that we get the master run id
* and the global offset, to try a partial resync at the next
* reconnection attempt. */
if (server.repl_state == REPL_STATE_SEND_PSYNC) {
if (slaveTryPartialResynchronization(conn,0) == PSYNC_WRITE_ERROR) {
err = sdsnew("Write error sending the PSYNC command.");
goto write_error;
}
server.repl_state = REPL_STATE_RECEIVE_PSYNC;
return;
}

/* If reached this point, we should be in REPL_STATE_RECEIVE_PSYNC. */
if (server.repl_state != REPL_STATE_RECEIVE_PSYNC) {
serverLog(LL_WARNING,"syncWithMaster(): state machine error, "
"state should be RECEIVE_PSYNC but is %d",
server.repl_state);
goto error;
}

psync_result = slaveTryPartialResynchronization(conn,1);

下面查看返回值

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if (psync_result == PSYNC_WAIT_REPLY) return; /* Try again later... */

/* If the master is in an transient error, we should try to PSYNC
* from scratch later, so go to the error path. This happens when
* the server is loading the dataset or is not connected with its
* master and so forth. */
if (psync_result == PSYNC_TRY_LATER) goto error;

/* Note: if PSYNC does not return WAIT_REPLY, it will take care of
* uninstalling the read handler from the file descriptor. */

if (psync_result == PSYNC_CONTINUE) {
serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Master accepted a Partial Resynchronization.");
if (server.supervised_mode == SUPERVISED_SYSTEMD) {
redisCommunicateSystemd("STATUS=MASTER <-> REPLICA sync: Partial Resynchronization accepted. Ready to accept connections.\n");
redisCommunicateSystemd("READY=1\n");
}
return;
}

/* PSYNC failed or is not supported: we want our slaves to resync with us
* as well, if we have any sub-slaves. The master may transfer us an
* entirely different data set and we have no way to incrementally feed
* our slaves after that. */
disconnectSlaves(); /* Force our slaves to resync with us as well. */
freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */

如果PSYNC能支持,我们前面就返回了,下面对于不支持的情况,我们就得用老的SYNC方法。在开始传输后,进入REPL_STATE_TRANSFER状态。

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/* Fall back to SYNC if needed. Otherwise psync_result == PSYNC_FULLRESYNC
* and the server.master_replid and master_initial_offset are
* already populated. */
if (psync_result == PSYNC_NOT_SUPPORTED) {
serverLog(LL_NOTICE,"Retrying with SYNC...");
if (connSyncWrite(conn,"SYNC\r\n",6,server.repl_syncio_timeout*1000) == -1) {
serverLog(LL_WARNING,"I/O error writing to MASTER: %s",
strerror(errno));
goto error;
}
}

如果不支持无盘加载,那么就要在磁盘上创建一个临时文件。
查看函数useDisklessLoad,无盘加载需要满足:

  1. repl_diskless_load配置
  2. 所有的模块都能处理读错误
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/* Prepare a suitable temp file for bulk transfer */
if (!useDisklessLoad()) {
while(maxtries--) {
snprintf(tmpfile,256,
"temp-%d.%ld.rdb",(int)server.unixtime,(long int)getpid());
dfd = open(tmpfile,O_CREAT|O_WRONLY|O_EXCL,0644);
if (dfd != -1) break;
sleep(1);
}
if (dfd == -1) {
serverLog(LL_WARNING,"Opening the temp file needed for MASTER <-> REPLICA synchronization: %s",strerror(errno));
goto error;
}
server.repl_transfer_tmpfile = zstrdup(tmpfile);
server.repl_transfer_fd = dfd;
}

下面非阻塞地进行SYNC,设置读取SYNC数据的回调readSyncBulkPayload,如果成功就切换状态为REPL_STATE_TRANSFER
这里,我们设置了repl_transfer_size为1,

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/* Setup the non blocking download of the bulk file. */
if (connSetReadHandler(conn, readSyncBulkPayload)
== C_ERR)
{
char conninfo[CONN_INFO_LEN];
serverLog(LL_WARNING,
"Can't create readable event for SYNC: %s (%s)",
strerror(errno), connGetInfo(conn, conninfo, sizeof(conninfo)));
goto error;
}

server.repl_state = REPL_STATE_TRANSFER;
server.repl_transfer_size = -1;
server.repl_transfer_read = 0;
server.repl_transfer_last_fsync_off = 0;
server.repl_transfer_lastio = server.unixtime;
return;

下面是错误处理,需要将状态重置为等待连接的REPL_STATE_CONNECT

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error:
if (dfd != -1) close(dfd);
connClose(conn);
server.repl_transfer_s = NULL;
if (server.repl_transfer_fd != -1)
close(server.repl_transfer_fd);
if (server.repl_transfer_tmpfile)
zfree(server.repl_transfer_tmpfile);
server.repl_transfer_tmpfile = NULL;
server.repl_transfer_fd = -1;
server.repl_state = REPL_STATE_CONNECT;
return;

write_error: /* Handle sendSynchronousCommand(SYNC_CMD_WRITE) errors. */
serverLog(LL_WARNING,"Sending command to master in replication handshake: %s", err);
sdsfree(err);
goto error;
}

slaveTryPartialResynchronization: PSYNC分支

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/* Try a partial resynchronization with the master if we are about to reconnect.
* If there is no cached master structure, at least try to issue a
* "PSYNC ? -1" command in order to trigger a full resync using the PSYNC
* command in order to obtain the master run id and the master replication
* global offset.
*
* This function is designed to be called from syncWithMaster(), so the
* following assumptions are made:
*
* 1) We pass the function an already connected socket "fd".
* 2) This function does not close the file descriptor "fd". However in case
* of successful partial resynchronization, the function will reuse
* 'fd' as file descriptor of the server.master client structure.
*
* The function is split in two halves: if read_reply is 0, the function
* writes the PSYNC command on the socket, and a new function call is
* needed, with read_reply set to 1, in order to read the reply of the
* command. This is useful in order to support non blocking operations, so
* that we write, return into the event loop, and read when there are data.
*
* When read_reply is 0 the function returns PSYNC_WRITE_ERR if there
* was a write error, or PSYNC_WAIT_REPLY to signal we need another call
* with read_reply set to 1. However even when read_reply is set to 1
* the function may return PSYNC_WAIT_REPLY again to signal there were
* insufficient data to read to complete its work. We should re-enter
* into the event loop and wait in such a case.
*
* The function returns:
*
* PSYNC_CONTINUE: If the PSYNC command succeeded and we can continue.
* PSYNC_FULLRESYNC: If PSYNC is supported but a full resync is needed.
* In this case the master run_id and global replication
* offset is saved.
* PSYNC_NOT_SUPPORTED: If the server does not understand PSYNC at all and
* the caller should fall back to SYNC.
* PSYNC_WRITE_ERROR: There was an error writing the command to the socket.
* PSYNC_WAIT_REPLY: Call again the function with read_reply set to 1.
* PSYNC_TRY_LATER: Master is currently in a transient error condition.
*
* Notable side effects:
*
* 1) As a side effect of the function call the function removes the readable
* event handler from "fd", unless the return value is PSYNC_WAIT_REPLY.
* 2) server.master_initial_offset is set to the right value according
* to the master reply. This will be used to populate the 'server.master'
* structure replication offset.
*/
  1. PSYNC_WRITE_ERROR 0
    套接口不可写。
  2. PSYNC_WAIT_REPLY 1
    需要read_erply设置为1,并调用函数。
  3. PSYNC_CONTINUE 2
  4. PSYNC_FULLRESYNC 3
    表示虽然支持PSYNC,但现在仍然需要一次Full SYNC。在这情况下,我们需要保存Master的runid和offset。
  5. PSYNC_NOT_SUPPORTED 4
    不支持PSYNC。
  6. PSYNC_TRY_LATER 5
    暂时连不上Master,要重试。
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int slaveTryPartialResynchronization(connection *conn, int read_reply) {
char *psync_replid;
char psync_offset[32];
sds reply;

首先,是写部分。这里的写,指的是往连接里面发送PSYNC指令:

  1. 如果我们缓存了server.masterserver.cached_master
    通常是在replicationCacheMaster中设置的
  2. 如果是第一次连
    发送
    1
    PSYNC ? -1
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/* Writing half */
if (!read_reply) {
/* Initially set master_initial_offset to -1 to mark the current
* master run_id and offset as not valid. Later if we'll be able to do
* a FULL resync using the PSYNC command we'll set the offset at the
* right value, so that this information will be propagated to the
* client structure representing the master into server.master. */
server.master_initial_offset = -1;

if (server.cached_master) {
psync_replid = server.cached_master->replid;
snprintf(psync_offset,sizeof(psync_offset),"%lld", server.cached_master->reploff+1);
serverLog(LL_NOTICE,"Trying a partial resynchronization (request %s:%s).", psync_replid, psync_offset);
} else {
serverLog(LL_NOTICE,"Partial resynchronization not possible (no cached master)");
psync_replid = "?";
memcpy(psync_offset,"-1",3);
}

/* Issue the PSYNC command */
reply = sendSynchronousCommand(SYNC_CMD_WRITE,conn,"PSYNC",psync_replid,psync_offset,NULL);
if (reply != NULL) {
serverLog(LL_WARNING,"Unable to send PSYNC to master: %s",reply);
sdsfree(reply);
connSetReadHandler(conn, NULL);
return PSYNC_WRITE_ERROR;
}
return PSYNC_WAIT_REPLY;
}

我们读出Master的回复,如果是空,我们就返回继续等待PSYNC_WAIT_REPLY

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/* Reading half */
reply = sendSynchronousCommand(SYNC_CMD_READ,conn,NULL);
if (sdslen(reply) == 0) {
/* The master may send empty newlines after it receives PSYNC
* and before to reply, just to keep the connection alive. */
sdsfree(reply);
return PSYNC_WAIT_REPLY;
}

connSetReadHandler(conn, NULL);

如果回复是+FULLRESYNC,表示需要一次Full SYNC。

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if (!strncmp(reply,"+FULLRESYNC",11)) {
char *replid = NULL, *offset = NULL;

/* FULL RESYNC, parse the reply in order to extract the run id
* and the replication offset. */
replid = strchr(reply,' ');
if (replid) {
replid++;
offset = strchr(replid,' ');
if (offset) offset++;
}

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    if (!replid || !offset || (offset-replid-1) != CONFIG_RUN_ID_SIZE) {
serverLog(LL_WARNING,
"Master replied with wrong +FULLRESYNC syntax.");
/* This is an unexpected condition, actually the +FULLRESYNC
* reply means that the master supports PSYNC, but the reply
* format seems wrong. To stay safe we blank the master
* replid to make sure next PSYNCs will fail. */
memset(server.master_replid,0,CONFIG_RUN_ID_SIZE+1);
} else {
memcpy(server.master_replid, replid, offset-replid-1);
server.master_replid[CONFIG_RUN_ID_SIZE] = '\0';
server.master_initial_offset = strtoll(offset,NULL,10);
serverLog(LL_NOTICE,"Full resync from master: %s:%lld",
server.master_replid,
server.master_initial_offset);
}
/* We are going to full resync, discard the cached master structure. */
replicationDiscardCachedMaster();
sdsfree(reply);
return PSYNC_FULLRESYNC;
}

否则,我们可以部分同步。

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if (!strncmp(reply,"+CONTINUE",9)) {
/* Partial resync was accepted. */
serverLog(LL_NOTICE,
"Successful partial resynchronization with master.");

/* Check the new replication ID advertised by the master. If it
* changed, we need to set the new ID as primary ID, and set or
* secondary ID as the old master ID up to the current offset, so
* that our sub-slaves will be able to PSYNC with us after a
* disconnection. */
char *start = reply+10;
char *end = reply+9;
while(end[0] != '\r' && end[0] != '\n' && end[0] != '\0') end++;

这里new表示Master端传来的runid。如果和我们当前的server.replid不一样,我们要重新设置一下,并且将老的server.replid复制给server.replid2
【Q】这里涉及到三个replid,他们的区别是什么呢?

  1. server.replid
  2. server.replid2
  3. server.cached_master->replid
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if (end-start == CONFIG_RUN_ID_SIZE) {
char new[CONFIG_RUN_ID_SIZE+1];
memcpy(new,start,CONFIG_RUN_ID_SIZE);
new[CONFIG_RUN_ID_SIZE] = '\0';

if (strcmp(new,server.cached_master->replid)) {
/* Master ID changed. */
serverLog(LL_WARNING,"Master replication ID changed to %s",new);

/* Set the old ID as our ID2, up to the current offset+1. */
memcpy(server.replid2,server.cached_master->replid,
sizeof(server.replid2));
server.second_replid_offset = server.master_repl_offset+1;

/* Update the cached master ID and our own primary ID to the
* new one. */
memcpy(server.replid,new,sizeof(server.replid));
memcpy(server.cached_master->replid,new,sizeof(server.replid));

如果当前Slave有Sub Slave,全部断开,让他们重新走PSYNC流程。

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                /* Disconnect all the sub-slaves: they need to be notified. */
disconnectSlaves();
}
}

/* Setup the replication to continue. */
sdsfree(reply);
replicationResurrectCachedMaster(conn);

/* If this instance was restarted and we read the metadata to
* PSYNC from the persistence file, our replication backlog could
* be still not initialized. Create it. */
if (server.repl_backlog == NULL) createReplicationBacklog();
return PSYNC_CONTINUE;
}

/* If we reach this point we received either an error (since the master does
* not understand PSYNC or because it is in a special state and cannot
* serve our request), or an unexpected reply from the master.
*
* Return PSYNC_NOT_SUPPORTED on errors we don't understand, otherwise
* return PSYNC_TRY_LATER if we believe this is a transient error. */

if (!strncmp(reply,"-NOMASTERLINK",13) ||
!strncmp(reply,"-LOADING",8))
{
serverLog(LL_NOTICE,
"Master is currently unable to PSYNC "
"but should be in the future: %s", reply);
sdsfree(reply);
return PSYNC_TRY_LATER;
}

if (strncmp(reply,"-ERR",4)) {
/* If it's not an error, log the unexpected event. */
serverLog(LL_WARNING,
"Unexpected reply to PSYNC from master: %s", reply);
} else {
serverLog(LL_NOTICE,
"Master does not support PSYNC or is in "
"error state (reply: %s)", reply);
}
sdsfree(reply);
replicationDiscardCachedMaster();
return PSYNC_NOT_SUPPORTED;
}

readSyncBulkPayload: SYNC分支 接受RDB

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/* Asynchronously read the SYNC payload we receive from a master */
#define REPL_MAX_WRITTEN_BEFORE_FSYNC (1024*1024*8) /* 8 MB */
void readSyncBulkPayload(connection *conn) {
char buf[PROTO_IOBUF_LEN];
ssize_t nread, readlen, nwritten;
int use_diskless_load = useDisklessLoad();
redisDb *diskless_load_backup = NULL;
int empty_db_flags = server.repl_slave_lazy_flush ? EMPTYDB_ASYNC :
EMPTYDB_NO_FLAGS;
off_t left;

/* Static vars used to hold the EOF mark, and the last bytes received
* form the server: when they match, we reached the end of the transfer. */
static char eofmark[CONFIG_RUN_ID_SIZE];
static char lastbytes[CONFIG_RUN_ID_SIZE];
static int usemark = 0;

/* If repl_transfer_size == -1 we still have to read the bulk length
* from the master reply. */
if (server.repl_transfer_size == -1) {
if (connSyncReadLine(conn,buf,1024,server.repl_syncio_timeout*1000) == -1) {
serverLog(LL_WARNING,
"I/O error reading bulk count from MASTER: %s",
strerror(errno));
goto error;
}

if (buf[0] == '-') {
serverLog(LL_WARNING,
"MASTER aborted replication with an error: %s",
buf+1);
goto error;
} else if (buf[0] == '\0') {
/* At this stage just a newline works as a PING in order to take
* the connection live. So we refresh our last interaction
* timestamp. */
server.repl_transfer_lastio = server.unixtime;
return;
} else if (buf[0] != '$') {
serverLog(LL_WARNING,"Bad protocol from MASTER, the first byte is not '$' (we received '%s'), are you sure the host and port are right?", buf);
goto error;
}

/* There are two possible forms for the bulk payload. One is the
* usual $<count> bulk format. The other is used for diskless transfers
* when the master does not know beforehand the size of the file to
* transfer. In the latter case, the following format is used:
*
* $EOF:<40 bytes delimiter>
*
* At the end of the file the announced delimiter is transmitted. The
* delimiter is long and random enough that the probability of a
* collision with the actual file content can be ignored. */
if (strncmp(buf+1,"EOF:",4) == 0 && strlen(buf+5) >= CONFIG_RUN_ID_SIZE) {
usemark = 1;
memcpy(eofmark,buf+5,CONFIG_RUN_ID_SIZE);
memset(lastbytes,0,CONFIG_RUN_ID_SIZE);
/* Set any repl_transfer_size to avoid entering this code path
* at the next call. */
server.repl_transfer_size = 0;
serverLog(LL_NOTICE,
"MASTER <-> REPLICA sync: receiving streamed RDB from master with EOF %s",
use_diskless_load? "to parser":"to disk");
} else {
usemark = 0;
server.repl_transfer_size = strtol(buf+1,NULL,10);
serverLog(LL_NOTICE,
"MASTER <-> REPLICA sync: receiving %lld bytes from master %s",
(long long) server.repl_transfer_size,
use_diskless_load? "to parser":"to disk");
}
return;
}

if (!use_diskless_load) {
/* Read the data from the socket, store it to a file and search
* for the EOF. */
if (usemark) {
readlen = sizeof(buf);
} else {
left = server.repl_transfer_size - server.repl_transfer_read;
readlen = (left < (signed)sizeof(buf)) ? left : (signed)sizeof(buf);
}

nread = connRead(conn,buf,readlen);
if (nread <= 0) {
if (connGetState(conn) == CONN_STATE_CONNECTED) {
/* equivalent to EAGAIN */
return;
}
serverLog(LL_WARNING,"I/O error trying to sync with MASTER: %s",
(nread == -1) ? strerror(errno) : "connection lost");
cancelReplicationHandshake(1);
return;
}
server.stat_net_input_bytes += nread;

/* When a mark is used, we want to detect EOF asap in order to avoid
* writing the EOF mark into the file... */
int eof_reached = 0;

if (usemark) {
/* Update the last bytes array, and check if it matches our
* delimiter. */
if (nread >= CONFIG_RUN_ID_SIZE) {
memcpy(lastbytes,buf+nread-CONFIG_RUN_ID_SIZE,
CONFIG_RUN_ID_SIZE);
} else {
int rem = CONFIG_RUN_ID_SIZE-nread;
memmove(lastbytes,lastbytes+nread,rem);
memcpy(lastbytes+rem,buf,nread);
}
if (memcmp(lastbytes,eofmark,CONFIG_RUN_ID_SIZE) == 0)
eof_reached = 1;
}

/* Update the last I/O time for the replication transfer (used in
* order to detect timeouts during replication), and write what we
* got from the socket to the dump file on disk. */
server.repl_transfer_lastio = server.unixtime;
if ((nwritten = write(server.repl_transfer_fd,buf,nread)) != nread) {
serverLog(LL_WARNING,
"Write error or short write writing to the DB dump file "
"needed for MASTER <-> REPLICA synchronization: %s",
(nwritten == -1) ? strerror(errno) : "short write");
goto error;
}
server.repl_transfer_read += nread;

/* Delete the last 40 bytes from the file if we reached EOF. */
if (usemark && eof_reached) {
if (ftruncate(server.repl_transfer_fd,
server.repl_transfer_read - CONFIG_RUN_ID_SIZE) == -1)
{
serverLog(LL_WARNING,
"Error truncating the RDB file received from the master "
"for SYNC: %s", strerror(errno));
goto error;
}
}

/* Sync data on disk from time to time, otherwise at the end of the
* transfer we may suffer a big delay as the memory buffers are copied
* into the actual disk. */
if (server.repl_transfer_read >=
server.repl_transfer_last_fsync_off + REPL_MAX_WRITTEN_BEFORE_FSYNC)
{
off_t sync_size = server.repl_transfer_read -
server.repl_transfer_last_fsync_off;
rdb_fsync_range(server.repl_transfer_fd,
server.repl_transfer_last_fsync_off, sync_size);
server.repl_transfer_last_fsync_off += sync_size;
}

/* Check if the transfer is now complete */
if (!usemark) {
if (server.repl_transfer_read == server.repl_transfer_size)
eof_reached = 1;
}

/* If the transfer is yet not complete, we need to read more, so
* return ASAP and wait for the handler to be called again. */
if (!eof_reached) return;
}

/* We reach this point in one of the following cases:
*
* 1. The replica is using diskless replication, that is, it reads data
* directly from the socket to the Redis memory, without using
* a temporary RDB file on disk. In that case we just block and
* read everything from the socket.
*
* 2. Or when we are done reading from the socket to the RDB file, in
* such case we want just to read the RDB file in memory. */
serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Flushing old data");

/* We need to stop any AOF rewriting child before flusing and parsing
* the RDB, otherwise we'll create a copy-on-write disaster. */
if (server.aof_state != AOF_OFF) stopAppendOnly();

/* When diskless RDB loading is used by replicas, it may be configured
* in order to save the current DB instead of throwing it away,
* so that we can restore it in case of failed transfer. */
if (use_diskless_load &&
server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB)
{
/* Create a backup of server.db[] and initialize to empty
* dictionaries */
diskless_load_backup = disklessLoadMakeBackups();
}
/* We call to emptyDb even in case of REPL_DISKLESS_LOAD_SWAPDB
* (Where disklessLoadMakeBackups left server.db empty) because we
* want to execute all the auxiliary logic of emptyDb (Namely,
* fire module events) */
emptyDb(-1,empty_db_flags,replicationEmptyDbCallback);

/* Before loading the DB into memory we need to delete the readable
* handler, otherwise it will get called recursively since
* rdbLoad() will call the event loop to process events from time to
* time for non blocking loading. */
connSetReadHandler(conn, NULL);
serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Loading DB in memory");
rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
if (use_diskless_load) {
rio rdb;
rioInitWithConn(&rdb,conn,server.repl_transfer_size);

/* Put the socket in blocking mode to simplify RDB transfer.
* We'll restore it when the RDB is received. */
connBlock(conn);
connRecvTimeout(conn, server.repl_timeout*1000);
startLoading(server.repl_transfer_size, RDBFLAGS_REPLICATION);

if (rdbLoadRio(&rdb,RDBFLAGS_REPLICATION,&rsi) != C_OK) {
/* RDB loading failed. */
stopLoading(0);
serverLog(LL_WARNING,
"Failed trying to load the MASTER synchronization DB "
"from socket");
cancelReplicationHandshake(1);
rioFreeConn(&rdb, NULL);
if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
/* Restore the backed up databases. */
disklessLoadRestoreBackups(diskless_load_backup,1,
empty_db_flags);
} else {
/* Remove the half-loaded data in case we started with
* an empty replica. */
emptyDb(-1,empty_db_flags,replicationEmptyDbCallback);
}

/* Note that there's no point in restarting the AOF on SYNC
* failure, it'll be restarted when sync succeeds or the replica
* gets promoted. */
return;
}
stopLoading(1);

/* RDB loading succeeded if we reach this point. */
if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
/* Delete the backup databases we created before starting to load
* the new RDB. Now the RDB was loaded with success so the old
* data is useless. */
disklessLoadRestoreBackups(diskless_load_backup,0,empty_db_flags);
}

/* Verify the end mark is correct. */
if (usemark) {
if (!rioRead(&rdb,buf,CONFIG_RUN_ID_SIZE) ||
memcmp(buf,eofmark,CONFIG_RUN_ID_SIZE) != 0)
{
serverLog(LL_WARNING,"Replication stream EOF marker is broken");
cancelReplicationHandshake(1);
rioFreeConn(&rdb, NULL);
return;
}
}

/* Cleanup and restore the socket to the original state to continue
* with the normal replication. */
rioFreeConn(&rdb, NULL);
connNonBlock(conn);
connRecvTimeout(conn,0);
} else {
/* Ensure background save doesn't overwrite synced data */
if (server.rdb_child_pid != -1) {
serverLog(LL_NOTICE,
"Replica is about to load the RDB file received from the "
"master, but there is a pending RDB child running. "
"Killing process %ld and removing its temp file to avoid "
"any race",
(long) server.rdb_child_pid);
killRDBChild();
}

/* Rename rdb like renaming rewrite aof asynchronously. */
int old_rdb_fd = open(server.rdb_filename,O_RDONLY|O_NONBLOCK);
if (rename(server.repl_transfer_tmpfile,server.rdb_filename) == -1) {
serverLog(LL_WARNING,
"Failed trying to rename the temp DB into %s in "
"MASTER <-> REPLICA synchronization: %s",
server.rdb_filename, strerror(errno));
cancelReplicationHandshake(1);
if (old_rdb_fd != -1) close(old_rdb_fd);
return;
}
/* Close old rdb asynchronously. */
if (old_rdb_fd != -1) bioCreateBackgroundJob(BIO_CLOSE_FILE,(void*)(long)old_rdb_fd,NULL,NULL);

if (rdbLoad(server.rdb_filename,&rsi,RDBFLAGS_REPLICATION) != C_OK) {
serverLog(LL_WARNING,
"Failed trying to load the MASTER synchronization "
"DB from disk");
cancelReplicationHandshake(1);
if (server.rdb_del_sync_files && allPersistenceDisabled()) {
serverLog(LL_NOTICE,"Removing the RDB file obtained from "
"the master. This replica has persistence "
"disabled");
bg_unlink(server.rdb_filename);
}
/* Note that there's no point in restarting the AOF on sync failure,
it'll be restarted when sync succeeds or replica promoted. */
return;
}

/* Cleanup. */
if (server.rdb_del_sync_files && allPersistenceDisabled()) {
serverLog(LL_NOTICE,"Removing the RDB file obtained from "
"the master. This replica has persistence "
"disabled");
bg_unlink(server.rdb_filename);
}

zfree(server.repl_transfer_tmpfile);
close(server.repl_transfer_fd);
server.repl_transfer_fd = -1;
server.repl_transfer_tmpfile = NULL;
}

/* Final setup of the connected slave <- master link */
replicationCreateMasterClient(server.repl_transfer_s,rsi.repl_stream_db);
server.repl_state = REPL_STATE_CONNECTED;
server.repl_down_since = 0;

/* Fire the master link modules event. */
moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
REDISMODULE_SUBEVENT_MASTER_LINK_UP,
NULL);

/* After a full resynchroniziation we use the replication ID and
* offset of the master. The secondary ID / offset are cleared since
* we are starting a new history. */
memcpy(server.replid,server.master->replid,sizeof(server.replid));
server.master_repl_offset = server.master->reploff;
clearReplicationId2();

/* Let's create the replication backlog if needed. Slaves need to
* accumulate the backlog regardless of the fact they have sub-slaves
* or not, in order to behave correctly if they are promoted to
* masters after a failover. */
if (server.repl_backlog == NULL) createReplicationBacklog();
serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Finished with success");

if (server.supervised_mode == SUPERVISED_SYSTEMD) {
redisCommunicateSystemd("STATUS=MASTER <-> REPLICA sync: Finished with success. Ready to accept connections.\n");
redisCommunicateSystemd("READY=1\n");
}

/* Send the initial ACK immediately to put this replica in online state. */
if (usemark) replicationSendAck();

/* Restart the AOF subsystem now that we finished the sync. This
* will trigger an AOF rewrite, and when done will start appending
* to the new file. */
if (server.aof_enabled) restartAOFAfterSYNC();
return;

error:
cancelReplicationHandshake(1);
return;
}

主事件循环

replicationCron

主要代码位于replication.c中。
主函数replicationCronserverCron触发,每隔一秒钟触发一次。

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run_with_period(1000) replicationCron();

下面查看主函数。

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// replication.c
void replicationCron(void) {
static long long replication_cron_loops = 0;

首先,下面是几个超时判断:

  1. 建立连接过程中超时
  2. 传输过程中超时
  3. 心跳/数据超时
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/* Non blocking connection timeout? */
if (server.masterhost &&
(server.repl_state == REPL_STATE_CONNECTING ||
slaveIsInHandshakeState()) &&
(time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
{
serverLog(LL_WARNING,"Timeout connecting to the MASTER...");
cancelReplicationHandshake(1);
}

/* Bulk transfer I/O timeout? */
if (server.masterhost && server.repl_state == REPL_STATE_TRANSFER &&
(time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
{
serverLog(LL_WARNING,"Timeout receiving bulk data from MASTER... If the problem persists try to set the 'repl-timeout' parameter in redis.conf to a larger value.");
cancelReplicationHandshake(1);
}

/* Timed out master when we are an already connected slave? */
if (server.masterhost && server.repl_state == REPL_STATE_CONNECTED &&
(time(NULL)-server.master->lastinteraction) > server.repl_timeout)
{
serverLog(LL_WARNING,"MASTER timeout: no data nor PING received...");
freeClient(server.master);
}

判断是否需要连接Master。
connectWithMaster这个函数会将状态设置为REPL_STATE_CONNECTING,并设置回调syncWithMaster

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/* Check if we should connect to a MASTER */
if (server.repl_state == REPL_STATE_CONNECT) {
serverLog(LL_NOTICE,"Connecting to MASTER %s:%d",
server.masterhost, server.masterport);
connectWithMaster();
}

如果Master支持PSYNC,就定期发送ACK。
这个ACK的作用是发送一个REPLCONF ACK命令给Master,从而通知自己当前的复制偏移。

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/* Send ACK to master from time to time.
* Note that we do not send periodic acks to masters that don't
* support PSYNC and replication offsets. */
if (server.masterhost && server.master &&
!(server.master->flags & CLIENT_PRE_PSYNC))
replicationSendAck();

下面,我们对所有Slave发送PING。根据注释,如果我们连接了Slave(是不是说明当前节点是Master?),就按时PING它们。这样Slave们能够维护到Master的显式的超时时间,从而在TCP连接并没有真正丢失的时候,检查一个断线的情况。

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/* If we have attached slaves, PING them from time to time.
* So slaves can implement an explicit timeout to masters, and will
* be able to detect a link disconnection even if the TCP connection
* will not actually go down. */
listIter li;
listNode *ln;
robj *ping_argv[1];

/* First, send PING according to ping_slave_period. */
if ((replication_cron_loops % server.repl_ping_slave_period) == 0 &&
listLength(server.slaves))
{
/* Note that we don't send the PING if the clients are paused during
* a Redis Cluster manual failover: the PING we send will otherwise
* alter the replication offsets of master and slave, and will no longer
* match the one stored into 'mf_master_offset' state. */
int manual_failover_in_progress =
server.cluster_enabled &&
server.cluster->mf_end &&
clientsArePaused();

if (!manual_failover_in_progress) {
ping_argv[0] = createStringObject("PING",4);
replicationFeedSlaves(server.slaves, server.slaveseldb,
ping_argv, 1);
decrRefCount(ping_argv[0]);
}
}

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/* Second, send a newline to all the slaves in pre-synchronization
* stage, that is, slaves waiting for the master to create the RDB file.
*
* Also send the a newline to all the chained slaves we have, if we lost
* connection from our master, to keep the slaves aware that their
* master is online. This is needed since sub-slaves only receive proxied
* data from top-level masters, so there is no explicit pinging in order
* to avoid altering the replication offsets. This special out of band
* pings (newlines) can be sent, they will have no effect in the offset.
*
* The newline will be ignored by the slave but will refresh the
* last interaction timer preventing a timeout. In this case we ignore the
* ping period and refresh the connection once per second since certain
* timeouts are set at a few seconds (example: PSYNC response). */
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;

int is_presync =
(slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
(slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
server.rdb_child_type != RDB_CHILD_TYPE_SOCKET));

if (is_presync) {
connWrite(slave->conn, "\n", 1);
}
}
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    /* Disconnect timedout slaves. */
if (listLength(server.slaves)) {
listIter li;
listNode *ln;

listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;

if (slave->replstate != SLAVE_STATE_ONLINE) continue;
if (slave->flags & CLIENT_PRE_PSYNC) continue;
if ((server.unixtime - slave->repl_ack_time) > server.repl_timeout)
{
serverLog(LL_WARNING, "Disconnecting timedout replica: %s",
replicationGetSlaveName(slave));
freeClient(slave);
}
}
}

/* If this is a master without attached slaves and there is a replication
* backlog active, in order to reclaim memory we can free it after some
* (configured) time. Note that this cannot be done for slaves: slaves
* without sub-slaves attached should still accumulate data into the
* backlog, in order to reply to PSYNC queries if they are turned into
* masters after a failover. */
if (listLength(server.slaves) == 0 && server.repl_backlog_time_limit &&
server.repl_backlog && server.masterhost == NULL)
{
time_t idle = server.unixtime - server.repl_no_slaves_since;

if (idle > server.repl_backlog_time_limit) {
/* When we free the backlog, we always use a new
* replication ID and clear the ID2. This is needed
* because when there is no backlog, the master_repl_offset
* is not updated, but we would still retain our replication
* ID, leading to the following problem:
*
* 1. We are a master instance.
* 2. Our slave is promoted to master. It's repl-id-2 will
* be the same as our repl-id.
* 3. We, yet as master, receive some updates, that will not
* increment the master_repl_offset.
* 4. Later we are turned into a slave, connect to the new
* master that will accept our PSYNC request by second
* replication ID, but there will be data inconsistency
* because we received writes. */
changeReplicationId();
clearReplicationId2();
freeReplicationBacklog();
serverLog(LL_NOTICE,
"Replication backlog freed after %d seconds "
"without connected replicas.",
(int) server.repl_backlog_time_limit);
}
}

/* If AOF is disabled and we no longer have attached slaves, we can
* free our Replication Script Cache as there is no need to propagate
* EVALSHA at all. */
if (listLength(server.slaves) == 0 &&
server.aof_state == AOF_OFF &&
listLength(server.repl_scriptcache_fifo) != 0)
{
replicationScriptCacheFlush();
}

replicationStartPendingFork();

/* Remove the RDB file used for replication if Redis is not running
* with any persistence. */
removeRDBUsedToSyncReplicas();

/* Refresh the number of slaves with lag <= min-slaves-max-lag. */
refreshGoodSlavesCount();
replication_cron_loops++; /* Incremented with frequency 1 HZ. */
}

如果SLAVEOF自己会怎么样?

Reference

  1. http://cbsheng.github.io/posts/redis%E5%AE%9A%E6%97%B6%E4%BB%BB%E5%8A%A1servercron/
  2. https://www.cnblogs.com/kismetv/p/9236731.html
  3. https://youjiali1995.github.io/redis/replication/
  4. https://wenfh2020.com/2020/05/31/redis-replication-next/
    有注释
  5. https://redis.io/commands/psync
  6. https://zhuanlan.zhihu.com/p/44105707
  7. https://zhuanlan.zhihu.com/p/86617437
    讲解sub slave